Permanent hair wave indicator



F. H. GULLIKSEN PERMANENT HAIR WAVE INDICATOR June; 2, 1942.

2 Shets-Sheet 1 Filed March 28, 1939 WITNESSES: W. 7W W ATTOR Patented June 2, 1942 PERMANENT HAIR WAVE INDICATOR Finn H. Gulliksen, Pittsburgh, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa.,-a corporation of Pennsylvania Application March 28, 1939, Serial No. 264,673

19 Claims.

My invention relates to electric discharge apparatus for use in producing a permanent wave or a curl in hair or the like.

While dry hair is an insulator, hair which is being waved or curled is saturated with an ammonia solution and is therefore conductive. During the waving process cellular reorientation of the hair takes place, and this manifests itself as a variation in the electrical resistance of the hair. The form of the hair is dependent on its cellular structure and therefore the character of the wave or curl ultimately produced in the hair is a function of the resistance of the hair at the instant when the heating is interrupted. Accordingly, by controlling the flow of heat to the hair in accordance with the resistance of the hair, a wave of any desired type may be produced.

As is well known, hair-waving establishments are frequented principally by persons of the feminine sex. Such persons are not only fastidious as regards their personal appearance, and in particular, their hair, but at times are also somewhat capricious. To satisfy clients of this type and to, as far as possible, avoid the consequences of their caprice, it is essential that the utmost care be taken in complying with their expressed or even unexpressed desires. When a hair waving treatment is completed, the hair, and even the owner of the hair must look precisely as she wants to look. Irregularities such as may result if one wave is loose and another is tieht may have disastrous consequences. For this reason, an arrangement for controlling the heat supplied for waving purposes must operate with the utmost precision and the final result must be fully predictable.

To complicate the problem of controlling the heat supplied for waving, the texture and com-- position of the hair of members of the fair sex varies over an extensive range. There are those whom nature has endowed as distinct blondes, brunettes. brownettes and red heads, but by far the greater proportion of their sisters fall by degrees between any two of these categories. In many cases, those dissatisfied with nature's gift introduce changes to suit themselves. Hair which has passed through an irreversible chemioal reaction so that its color and composition is changed, is referred to as having been touched up. This expression is an understatement.

It is apparent that under the circumstances, the owner of a hair waving establishment finds his position no bed of roses. The resistance of hair which is determinative in hair waving varies widely, not only from person to person, b t for the same individual. A person whose hair has, say, a high electrical resistance one week, because she happens to be blonde, may, the next week, in eflect, short-circuit the control equipment, because she has, during the week, become dissatisfied with the color of her hair and has changed it. While the wave produced is substantially dependent on the relative change in resistance rather than the absolute resistance values, changes in the texture of hair do introduce difliculties. Hence, any arrangement designed to be used for varying the heating of hair in accordance with electrical resistance, or even the relative change in electrical resistance, to produce a wave or a curl, must eliminate the variables introduced by changes in the hair.

It is accordingly an object of my invention to provide an arrangement, functioning in dependence on the resistance of hair to be waved or curled, to control the supply of heat to the hair, that shall produce the precise wave desired.

Another object of my invention is to provide an arrangement for controlling the supply of heat to hair that is to be waved, that shall function in a precisely predictable manner.

A further object of my invention is to provide an arrangement for use in supplying heat to produce a wave or a curl in hair that shall operate to produce a desired wave independently of variations'in the electrical resistance of the hair.

Still another object of my invention is to provide a hair waving control arrangement, by the operation of which the uniform waves or curls shall be produced in the hairtreated.

An ancillary object of my invention is to provide an arrangement that shall respond to a potential, which varies as a function of time and may assume the same value at different instants, to operate in a predetermined manner after the potential has attained the said value a predetermined number of times.

Another ancillary object of my invention is to provide an arrangement, responsive to a potential which decreases to a minimum and then increases, that shall operate in a predetermined manner when the potential has increased to a predetermined value.

More concisely stated, it is an object of my invention to provide a simple and inexpensive system for controlling the heat supplied to hair being waved or curled that shall function with precision and that shall produce completely predictable results.

When hair is heated the resistance of the hair'expressed in terms of the heating time is a function which has a minimum at a finite heating temperature. The instantaneous values of the function are determinative for the production of a curl or a wave in hair. However, the values of the function having any significance do not lie along its falling branch but a the asymmetric device.

along its rising branch. Since the hair passes through the significant resistance-values along the falling branch of the resistance-time function before it attains the values along the rising branch of the curve, the principal problem involved on my invention is to prevent the operation of the controlling apparatus in a hair treating system until the significant resistance values are reached for the second time.

According to my invention, the supply of heat to hair is controlled by an electric discharge valve, preferably, but not necessarily, of the arclike type. In the control circuit of the valve a capacitor and a resistor are provided. The capacitor and resistor are connected in an auxiliary circuit in which an asymmetrically conductive device such as an electric discharge rectifier or a dry rectifier is interposed. Current may, therefore, flow between the capacitor and the resistor in one sense only, and when the capacitor attains a predetermined potential by reason of current flow through the resistor, in one sense, .a change in the capacitor potential is prevented by A potential which depends on the resistance of the hair is impressed across the resistor.

While the resistance of hair is decreasing during the heating process, the potential of the capacitor is changed by charge or discharge through the asymmetric device and the change is counteracted by the potential impressed on the resistor. After the hair resistance has passed through the minimum the counteracting effect of the potential dependent on the hair resistance is reversed and at the same time capacitor potential is prevented from change by the asymmetric device. \Vhen the hair now increases to the significant resistance, the corresponding potential produces a change in the conductivity of the valve and causes the control equipment to operate.

Variations in the resistance of the hair are prevented from materially influencing the character of the wave produced, in accordance with another aspect of my invention, by so arranging the capacitor and resistor circuit that the counteracting potentials impressed on the resistor increases, and decreases in direct proportion to the hair resistance. This object is accomplished by charging the capacitor before the treating operation. During the treating operation the capacitor is discharged and a potential, proportional to the hair resistance, is impressed across the resistor in such manner that its polarity is opposite to the polarity of. the capacitor potential and counteracts the effect of the discharge. When the hair attains its minimum resistance, the capacitor attains its minimum potential. As the potential proportional to the hair resistance now increases, it becomes effective to operate the control arrangement, because the capacitor is prevented by the asymmetric device from having compensating effect.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. tion itself, however, both as to its organization and its method of operation, to ether with additional objects and advantages thereof, will best be understood from; the following description of specific embodiments when read in connection with the accompanying drawings in which:

The "inven- Figure 1 is a graph illustrating how the retime,

Fig. 2 is a diagrammatic view showing an embodiment of my invention and Fig. 3 is a diagrammatic view showing a modification of my invention.

In Figure l, the resistance of the hair is plotted vertically in ohms: heating time is plotted horizontally in arbitrary units. Two curves 5 and I to represent the hair resistance of two different people, or possibly of one person at difi'erent times are reproduced. As can be seen from the curves, the hair resistance decreases from a value of the order of a thousand ohms to several hundred ohms or even less, and then increases. My invention depends upon the realization that the character of the wave or curl produced is determined by the electrical resistance of the hair at the instant when the heating is interrupted. As indicated, three points marked L, M, and T may be selected to correspond to a loose wave, a medium wave, and a tight wave. Since the hair resistance significant for a hair wave of any type, exists at two different times and the control apparatus must be operated when it appears the second time, the principal problem involved is to provide a control system which shall fall to respond when the resistance value appears the first time, but shall respond when it appears the second time.

This object may be accomplished with a system such as is shown in Fig. 2. The hair 3 is saturated with an ammonia solution, secured near the scalp in clamps I0 and wound in a number of conductive tubes H. When the hair is wound on the tube II, a ribbon l2 of conductive foil is disposed along it in intimate contact with it and is also wound about the tube II. The tube H is secured to the bracket of the clamp l2 and insulated therefrom and is covered with a water-saturated cloth (not shown for purposes of clarity). An electrical heater I3 is disposed over the tube H and the hair. The heater I3 is energized from a secondary section M of the supply transformer IS. The tube II and the foil l2 are connected across the terminals of a secondary section I! of the transformer 45, through a full wave rectifier IS. The direct current terminals of the rectifier 19 are connected to the saturating winding 2| of a saturable reactor 23 and the impedance of the reactor is thus varied in accordance with the resistance of the hair 9.

The alternating current windings 25 of the reactor 23 are connected across another secondary section 21 of the transformer i1, through the primary 29 of an auxiliary transformer 3!. The secondary 33 of the latter transformer charges an auxiliary capacitor 35 through a second full-wave rectifier 31. The capacitor 35 is connected in the control circuit of a discharge valve 39 of the high vacuum type and controls the flow of current between the anode ll and cathode 43 of the valve. Initially, the resistance of the hair is relatively high and impedance of the reactor 23 is correspondingly large and the charge on the capacitor 35 is small. As the heating progresses, the reactor impedance decreases as the hair resistance decreases, and the charge impressed on the capacitor increases. The capacitor 35'is so connected in the control circuit of the high vacuum valve 39 that, as its potential increases, the current flow through the valve increases.

The valve 39 is connected in a circuit which extends from the positive terminal 43 of a voltage divider 41 supplying direct current through an auxiliary voltage divider 89, the anode II and cathode 43 of the high vacuum valve 88, to an intermediate tap 8| of the main voltage divider. The capacitor 85 is connected between the negative terminal 53 of the voltage divider 4! and the control electrode 55 of the valve 39 and the potential, dependent on the hair resistance which is impressed On it. counteracts the bias potential provided by the voltage divider.

The auxiliary voltage divider 49 is connected in a series circuit including a pair of capacitors 51 and 59 and a rectifier 8| preferably of the high vacuum type. The potential drop across the latter voltage divider 49 produced by the current fiow through the high vacuum valve 39 is such that the capacitors 51 and 59 are charged through the rectifier 8|. The adjustable tap 88 of the auxiliary voltage divider I9 is connected to the cathode 65 of an are like valve 61 through a portion of a second voltage divider 59 which serves to supply a direct current bias potential to the arc-like valve 51. The control electrode II of the arc-like valve is connected intermediate the two capacitors 51 and 59 and the anode 13 of the arc-like valve is connected to the positive terminal 15 of the second voltage divider 69 through the exciting coil "of a relay I9 and the upper contact 8| of a reset switch 83.

Initially the bias provided by the second voltage divider 89 maintains the arc-like valve 81. non-conductive. The charge impressed on the capacitors 51 and 59 by reason of the potential impressed across the auxiliary divider 49 is in such a sense as to counteract the bias potential. However, as the current flow through the high vacuum valve 39 increases by reason of the decrease in the resistance of the hair 9, the effect of the capacitor potential is counteracted by the potential drop across the auxiliary voltage divider 49 in series with the high vacuum valve 39 and the arc-like valve 61 remains non-conductive. When the hair resistance attains its minimum value, the capacitors 51 and 59 reach their maximum potential. After this the current flow through the voltage divider l9 and the high vacuum valve 39 decreases and the potential drop across the voltage divider becomes smaller than the potential of the'capacitors. The discharge of the capacitors 51 and 59 into the auxiliary divider 49 is, however, prevented by the rectifier GI and, therefore, the capacitor potential for minimum hair resistance remains impressed in the control circuit of the arc-like valve 61 while the potential drop across the voltage divider by which it was originally counteracted, continues to decrease. At a point predetermined by the setting of the adjustable tap 83 of the auxiliary voltage divider 49, the critical control potential of the arc-like valve 61 is exceeded and the latter is rendered conductive.

The relay I9 is now operated and actuates its contactor 85 to open the supply circuit 81 of the heater I3. As a result the heating of the hair 9 is interrupted. The setting of the tap 63 of the voltage divider 49 is such that the interrupter corresponds to one or the other of the points L, M or T on a curve, such as or I, corresponding to the hair. When the hair 9 is removed from the container I3, it therefore has the desired wave. The arrangement can be reset by operating the switch 83. The resetting operation may be accomplished by momentarily actuating switch 83 to close the lower contact members and to open upper contact members 8| and then restoring the contact members of switch 88 to their original condition as shown in Fig. 2. The closing of the lower contact members of switch 83 completes a circuit for discharging capacitors 51 and 58. The opening of contact members 8| deenergizes valve 81 and coil ll of relay 19, thereby reclosing contact members to reclose the supp y circuit 81 of heater I3 and to initiate the next cycle of operation similar to that hereinbefore explained. Coil 11 remains deenergized at the beginning of the cycle of operation when contact members 8I are reclosed because of the deionization of valve 81.

The operation of the apparatus disclosed in Fig. 2 may, of course. be made dependent on the.

absolute resistance of the hair. For this purpose a set of characteristic curves for diil'erent initial hair resistances may be provided with each waving machine and the setting for any client determined from the curve after the initial resistance of the hair has been measured. However, it has been found that the characteristics of a wave depends on the ratio of the hair resistance at which the heating is interrupted to the minimum resistance and this principle may be applied to facilitate the operation. x

In the apparatus disclosed in Fig. 2, difllculties are involved in properly setting the apparatus because the potential on which the operation of the control equipment is dependent varies in the opposite sense to the resistance of the hair 9. The high vacuum valve 39 is operated along the linear portion of its characteristic and therefore the potential impressed across the auxiliary voltage divider 49 is proportional to the potential impressed on the capacitor 35. For proper operation the latter potential must vary in direct proportion to the reciprocal of the hair resistance. Unless this relationship is maintained, the ratio of the hair resistance at which the operation takes place to the minimum resistance is different for any setting of the divider 49 and for different initial or minimum hair resistances. The hyperbolic relationship between potential and hair resistance which is necessary for proper operation is sometimes difficult to obtain, and for this reason, I have provided the arrangement shown in Fig. 3 in which it is not necessary.

In this case the hair resistance 89 shown symbolically is interposed between the control electrode 9| of a high vacuum valve 93 and one terminal of the secondary of an auxiliary transformer 91 supplied from an alternating current source 99 which may be of the usual 60 cycle type. The cathode IIH of the high vacuum valve 93 is connected to the intermediate tap I93 of the secondary 95. The remaining terminal of the secondary 95 is connected through a resistor I05 to the junction print of the hair resistance 89 and the conductor from the control electrode 9|. The resistor I85 may, for the purpose of the present explanation, be assumed to have the magnitude of the initial hair resistance. However, it may, and often does, have a magnitude equal to the average hair resistance or any other magnitude within the range through which the hair resistance varies. I have used a resistor Hi5 having a resistance of 500 ohms. This resistance corresponds to a hair resistance range from to 1200 ohms. The common connection point of the intermediate tap I83 and the cathode I0! is also connected to the junction point I01 of two networks I99 and III. Each of the networks I09 and III consists of a resistor I I3 shunted by a capacitor 5. The terminals III of the networks I99 and III that are not connected together are connected, respec tively, to opposite polarity terminals of. corresponding secondary sections H9 and III of the main supply transformer I23. The remaining terminals of the secondary sections H9 and HI are connected to the anodes I25 of a full-wave rectifier tube I21. The cathode I 29 of the rectifier I21 is connected to the anode HI the high vacuum valve 93. Since the secondary sections H9 and I 2| are connected with their opposite polarity terminals to the networks III9 and III and to the anodes I25, current is conducted during alternate halt periods of the source through each or the resistors H3 in the networks, and the capacitors III are correspondingly charged.

The hair resistance 339 is connected in a balanced network of whlchit forms one arm, the

resistor I05 in series with it forms the other arm, and the remaining arms .are constituted by the sections of the secondary 95 of the auxiliary transformer 91 eitending from the intermediate Initially, when the hair is cold,-the*networl I is substantially balanced and the potential between the control electrode 9| and the cathode I III is zero. At this time. the current fiow through the resistors I, I3 in series with the rectifler I 21 during the alternate half-periods is equal, and the capacitors II5 are charged to substantially the same potentials. As the hair resistance 99 decreases, an increasing alternating potential is impressed between the control electrode 9I and the cathode I III. The current flow through one of the series resistors H3, is, there- 'iore, correspondingly increased while the current flow through the other is correspondingly decreased. The parallel capacitors II5 are, therefore, charged to increasingly. difierent potentials. y

The potential drop across the two capacitors H5 is impressed on another capacitor I33 which The cathode II of the high vacuum these elements varies in substantially direct proportion with the hair resistance.

Across the resistor I59 and the voltage divider I51, a capacitor I9I is connected in series with a rectifier. I53. The capacitor I5I is connected in the control circuit of an arc-like valve I55 through which the relay I51 controlling the heat supply to the hair is energized. The control circuit extends from the upper terminal of the capacitor IIiI through a conductor I99, a biasing .button or a foot switch, the capacitor I9I, the

lower contact I35 0! the circuit controller, to the negative terminal I49 of the supply; When the treatment is initiated, the foot switch is operated and the charging circuit for the capacitor I5I is opened. The capacitor potential is initially substantially equal to the potential drop across the direct current supply I41. The potential impressed across the high vacuum valve I39 and the two series impedances I51 and I59 is also substantially equal to that-oi the direct current supply.

} across the high vacuum valve I39. The rectifier I53 is so connected in series with the capacitor I5I and the voltage divider I51 that current may I 'ilowirom the capacitor, through the. voltage the voltage divider and resistance.

is connected across the unconnected, terminals capacitor is connected between the control eleca trode I35 and the cathode I31 of a second high vacuum valve I 39 through resistors I and I43, respectively.v Another capacitor I is connected directly between the control electrode I35 and the cathode I31.

The high vacuum valve I39 is supplied with direct current anode-cathode potential derived from the main source 99 through a rectifier system I41. The negative terminal I49 oi the direct current-supply is connected to the junction point I5I of the capacitor I33 and the resistor I43 in series with the cathode I31. The positive terminal I53 of the supply I41is connected to the anode I55 of the high vacuum'valve I39 through a voltage divider I 51 and a resistor I 59. The connection of the capacitors I33 and I45 to the networks I99and III in the hair resistance circuit is such that the potential impressed between the control electrode I35 and the cathode I 31 of the valve I39 is directly proportional to the hair resistance; The valve I39 is operated along the linear portionof its characteristic and, therefore, the current fiow through the resistor I 59 and. the voltage divider I51in series-with the valve and the potential drop across divider I51 and the resistor I59'when the capacitor potential is larger than the potential across Initially, therefore, the capacitor potential automatically adjusts itself to correspond to the potential drop across the voltage divider and the resistor.

The plate oi the capacitor which is connected to the cathode'I15 oi the arc-like valve I55 is positive and the other plate is negative. The capacitor potential, therefore, tends to maintain the valve I55 non-conductive when the switch I83 is, first operated. As the treatment progresses the current fiow through the high vacuum valve I39 decreases by reason oi-the decrease'in the hair ,resistance99, the potential drop acrossthe' voltage divider I51 and resistor I59 decreases and the capacitor I5I continues to discharge through the two elements However, the decrease in potential 01' the capacitor I9I has no eflect on the arc-like valve I55 as it is counteracted by the potential drop arising from the current fiow. through the resistance I59 and the voltage divider I51. 9

After the hair attains the minimum resistance, the potentialacross the voltage divider I51 and resistor I59 increases. At this point the potential impressed on the capacitor IBI is a minimum and the rectifier I 53 in series with the capacitor prevents the latter from being recharged by reason of the, potential drop across the resistance I59 and the divider I51. As a result, the positive potential impressed between the control electrode I11 and the cathode I15 of the arc-like valve I gradually increases by reason .0! the increasing current flow through the high vacuum valve I39. The critical potential of the valve is exceeded at a value of hair resistance corresponding to the setting of the adjustable tap I19 of the voltage divider. At this point the supply oi current to heat the hair is discontinued. Since potential across the resistor I59 and voltage divider I] is in this case proportional to the hair resistances, the ratio of the hair resistance at the point of operation to the minimum hair resistance for any setting of the tap I19 is independent of the absolute hair resistance.

Although I have shown and described certain specific embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except in so far as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. For use in the heating of hair or the like from a source of current to produce a wave or curl of a predetermined character therein the combination comprising a circuit for supplying said hair with current from said source, electric discharge valve means for controlling the magnitude of the current flow through said circuit and means responsive to the electrical resistance of said hair for controlling said valve means.

2. For use in the heating of hair or the like from a source of current to produce a wave or curl of a predetermined character therein the combination comprising a circuit for supplying said hair with current from said source, means for controlling the magnitude of the current flow through said circuit and means responsive to the electrical resistance of said hair for controlling said valve means, said responsive means including a saturable reactor having a saturating winding supplied with current which varies in accordance with the resistance of the hair.

3. For use in the heating of hair or the like from a source of current to produce a wave or curl of a predetermined character the combination comprising a circuit for supplying said hair with current from said source, an electric discharge valve having a control circuit for controlling the magnitude of the current fiow through said circuit, means for impressing a potential in said control circuit which responds to the minimum resistance of the hair, and means functioning to counteract said potential until said hair attains a predetermined resistance greater than the minimum. V

4. For use in the heating of hair or the like from a source of current to produce a wave or curl of a predetermined character the combination comprising a circuit for supplying said hair with current from said source, an electric discharge valve having a control circuit for controlling the magnitude of the current flow through said circuit, a capacitor and resistor in 'said control circuit, means for charging said capacitor in response to the resistance of said hair and in such a sense that the potential on said capacitor tending to render said valve conductive increases as the reactance of the hair decreases, said charging means including said resistor and means for preventing the discharge of said capacitor, said resistor being so connected that the potential drop across it during the charging counteracts the effect of said capacitor potential on said valve.

5. In combination, an electric discharge valve having a control electrode and a principal electrode, a network including charge storing means, a resistor and asymmetric conductive means interposed between said resistor and said charge storing means, means for connecting said charge-storing means to one of said electrodes, means for connecting said resistor to the other of said electrodes and means for supplying current of varying magnitude through said resistor.

6. In combination, an electric discharge valve having a control electrode and a principal electrode, a network including charge storing means, a resistor and asymmetric conductive means interposed between said resistor and said charge storing means, means for connecting said charge storing means to one of said electrodes, means for connecting said resistor to the other of said electrodes and means for supplying current the magnitude of which decreases to a predetermined minimum value and then increases from said minimum value through said resistor.

7. For use in supplying current from a source to heat a material the resistance or which varies in magnitude as a function of the heating time from a predetermined value to a minimum and then to a second value higher than said minimum, the combination comprising a circuit for supplying said material from said source, electric discharge valve means for controlling the magnitude of the current flow through said circuit and means responsive to the resistance of said material for controlling said electric discharge valve means.

8. For use in supplying current from a source to heat a material the resistance of which varies in magniture as a function of the heating time from a predetermined value to a minimum and then to a second value higher than said minimum, the combination comprising a circuit for supplying said material from said source, electric discharge valve means of the arc-like type for controlling the closing and opening of said circuit and means responsive to the resistance of said material for controlling said electric discharge valve means.

9. In combination an electric discharge valve having a control electrode and a principal electrode, a capacitor, a discharge circuit for said capacitor including an impedance and an asymmetrically conductive device, means for connecting said circuit and said capacitor between said control electrode and said principal electrode, means for charging said capacitor to a potential such that said valve is initially non-conductive, and means for impressing a potential across said impedance which decreases from a value at most equal to that of the potential to which said capacitor is charged to a minimum value and then increases.

10. In combination an electric discharge valve having a control electrode and a principal electrode, charge storing means, a discharge circuit for said charge storing means including an impedance and an asymmetrically conductive device, that permits said charge storing means to discharge when the potential across said charge storing means is greater than the potential across said impedance, means for connecting said circuit and said charge storing means between said control electrode and said principal electrode, mean for charging said charge-storing means to a potential such that said valve is initially non-conductive, and means for impressing a potential across said impedance which decreases from a value at most equal to that of the po tential to which said charge storing means is charged to a minimum value and then increases.

11. In combination an electric discharge valve having a control electrode and a principal electrode, charge-storing means, means for charginglarity in the resistor-charge-storing means circuit-to the potential oi said charge-storing means and which varies from a vaiueat most equal to the initial potential on said charge-storing means to a minimum value and then increases to a value such that said valve is rendered conductive.

12. In combination an electric discharge valve having a control electrode and a principal electrode, charge-storing means, means for charging said charge-storing means, a resistor and an asymmetrically conductive device so connected to said charge-storing means that it can discharge through said resistor, means !or connecting said charge-storing means and said resistor in circuit with, said control electrode and principal electrode in such manner that said charge-storing means when charged maintains.

said valve non-conductive and means for impressing a varying potential across said resistor, while said charge-storing means is discharging.

13. For use with an element the impedance oi! which, when expressed in terms of time over a predetermined'interval, is a function which has a minimum at a flnite'time point in said interval, the combination comprising an electric discharge valve having a control electrode and a principal electrode, charge-storing means, means .ior charging said charge-storing means, a resistor and an asymmetrically conductive deviceso connected to said charge storing means that it can discharge through said resistor, means for connecting said charge-storing means and said resistor in circuit with said control electrode and principal electrode in such manner that said charge-storing means when charged maintains said valve non-conductive and means for impressing a potential across said resistor, while said charge-storing means is discharging, which is of opposite polarity in the resistor-chargestoring means circuit to the potential of said charge-storing means and which varies in the same sense as the impedance of said element varies during said interval.

. 14. In combination an electric discharge valve having a control electrode and a principal electrode, charge-storing means, means for charging said charge-storing means, a resistor and an asymmetrically conductive device so connected to said charge-storing means that it can discharge through said resistor, means for connecting said charge-storing means and said resistor in circuit with said control electrode and principal electrode and means for impressing a potential across said resistor, while said charge-storing means is discharging, which is of opposite polarity in the resistor charge-storing means circuit to the potential of said charge-storing means and which varies from a value at most equal to the initial potential on said charge-storing means to a minimum value and then increases to a value such that saidvalve is rendered conductive.

15. In combination an electric discharge valve having a control electrode and a principal electrode, charge-storing means, a discharge circuit for said charge-storing means including an impedance and an asymmetrically conductive device that permits said charge-storing means to discharge when the potential across said charge-storing means is greater than the. potential across said impedance. means for connecting saidcircuitand said charge-storing means between said control electrode and said principal electrode, means for charging said charge-storing means to a predetermined potential and means for impressing a potential across said impedance which decreases from a value at must equal to that of the potential to which said charge-storing means is charged to a minimum value and then increases.

, 16. For use with an element the-impedance of which, when expressed in terms of time over a predetermined interval, is a function which has cuit for said capacitor including an impedance,

and an asymmetrically conductive device, means for connecting said circuit and said capacitor between said control electrode and said principal electrode, means for charging said capacitor to a predetermined potential and means for impressing a potential across said impedance which varies in the same manner as said impedance during said interval. 1

17. In combination, an electric discharge valve having a control electrode and a principal electrode, a network including charge-storing means, a resistor and asymmetric conductive means interposed between said resistor and said chargestoring means, means for connecting said chargestoring means to one of said electrodes, means for connecting said resistor to the other of said electrodes and means {or supplying current the magnitude oi which first varies in one sense and controlling said electric discharge valve means.

19. In combination, an electric discharge valve having acontrol electrode and a principal electrode, a network including a resistor, chargestoring means and asymmetric conductivemeans connected in-series circuit relation across said resistor,

means for connecting said charge-I storing meansto one of said electrodes, means for connecting said resistor to the other of said 

